1. Field of the Invention
The present invention relates to improvement of an encoder which utilizes coherent light.
2. Description of the Related Art
FIG. 1 shows a conventional encoder which utilizes coherent light. This encoder is comprised of a main body 7 and a scale 9. The main body 7 incorporates a semiconductor laser 1 (which has one light beam-output terminal), a lens 2, a half mirror 3, reflecting mirrors 4 and 5, and photodetector 6. The scale 9 is provided with a large number of gratings 8. A typical example of such an encoder is disclosed in U.S. Pat. Nos. 4,868,385 4,883,955 and 4,899,048.
A light beam 10 output from the output terminal of the semiconductor laser 1 is converged by the lens 2, and is then divided into two light beams 11 and 12 by the half mirror 3. Light beams 11 and 12 are reflected by reflecting mirrors 4 and 5, respectively, such that the reflected light beams intersect each other. Thereafter, the light beams 11 and 12 are incident on the scale 9. The light beams 11 and 12 are diffracted by the gratings 8, and therefore become diffracted light beams 13 and 14, respectively. These diffracted light beams 13 and 14 interfere with each other, and become a coherent light beam. This coherent light beam is incident on the photodetector 6, for measurement of the beam intensity.
Either the main body 7 or the scale 9 is attached to a given movable object (not shown). When the object moves, the intensity of the coherent beam changes in proportion to the distance for which the object moves. Thus, the photodetector 6 produces a signal which changes in proportion to that distance. This being so, the encoder can be used as a sensor which is to be incorporated in a micro machine.
In the above encoder, the light beam is output from only one end of the semiconductor laser 1. Due to this structure, not only the semiconductor laser 1 and the photodetector 6 but also the lens 2, the half mirror 3 and the reflecting mirrors 4 and 5 have to be positioned and fixed with high accuracy. The encoder incorporating all these structural components is inevitably large in size. In addition, it takes time to position the optical components. Further, since the accuracy with which the structural components are positioned is likely to be degraded by external vibration, the encoder does not always ensure high measurement accuracy.